The present invention generally relates to a deflection control ink jet printing apparatus of the type which ejects ink under pressure from a nozzle, applies vibration to the ejected ink to form ink droplets regularly, develops a multi-level electric field selectively in accordance with an image signal when each ink droplet shapes itself, charges ink droplets by the electric field, and deflects charged ink droplets by a deflecting electric field to a multiplicity of levels. More particularly, the present invention is directed to a charge level setting device which can automatically set and adjust a charging level to promote adequate deflection of charged ink droplets.
In such a multi-value or multi-level deflection control type ink jet printer, one nozzle is alotted to cover three or more picture elements (e.g. 40 dots in a width of 5 mm, assuming 8 dots/mm). Printing ink droplets are charged to three or more levels (e.g. 40 levels) to be deflected along three or more different paths (e.g. 40 paths). A recording medium in the form of a paper sheet is placed at a relatively large distance from a nozzle of the printer. With this in view, ink is pressurized to a level high enough for a droplet thereof from the nozzle to reach the recording sheet in a stable manner along a predetermined path, despite its passage through the charging and deflecting electric fields. In order that ink droplets of a given diameter may appear regularly and follow their predetermined paths accurately, it is a prerequisite that a variety of factors such as a viscosity and pressure of ink, a vibrating pressure, an amount of charge and an intensity of electric field for deflection be stabilized and controlled exactly.
However, it is impossible in practice to hold all such factors under fully controlled conditions. A known ink jet printer has a plurality of ink ejection holes arranged such that a string of ink droplets from each ejection hole are charged to several different levels to print dots in different positions on a paper sheet and, thereby, reproduce an image continuously together with those ejected from the adjacent holes. In this type of ink jet printer in particular, the impracticality of the ideal control over the various factors may cause the droplets from adjacent holes to become spaced from said other, overlapped each other or otherwise misdeflected. Where a printer head having a single ejection hole is moved in a direction perpendicular to the direction of deflection, the maximum width of deflection becomes either expanded or contracted.
These problems have hitherto been settled by an expedient in which an actual deflection position of ink droplets of a given deflection step in detected, a charging voltage is so adjusted as to permit said ink droplets to reach a predetermined reference position, and then charging voltages for the other deflection steps are computed from the thus determined charging voltage.
Another cause of misdeflection of an ink droplet is the distortion of its charge attributable to the charges on ink droplets which are flying just a head of it. In detail, image signals have two different levels, "1" and "0", regularly or randomly in accordance with original images while ink droplets are selectively charged and non-charged corresponding to the "1" and "0" signal levels. An electric field developed by the charges on one to four preceding droplets affect a droplet which is about to be charged; the following droplet will be deposited with a less charge than would be deposited by the actual voltage without any disturbance. It is therefore desirable that any distortion in deflection be compensated for in accordance with the charges on the preceding droplets and the distribution of flying droplets.